The present invention relates generally to a power transmission system for motor vehicles, which changes automatically engine output rotation and transfers it to driving wheels, and more particularly, to a power transmission system for four-wheel drive vehicles, which distributes power to front and rear wheels.
Typically, a power transmission system for four-wheel drive vehicles with a toroidal continuously variable transmission (CVT) comprises a power distributing device provided to the toroidal CVT on the rear side of the vehicle and for distributing to the rear and front wheels rotation obtained by converting rotation input from the engine.
However, this power transmission system raises the following problems. Since the power distributing device is linked to the toroidal CVT in the unit to share the use of oil, the device has a large friction loss. Specifically, due to its structure of transferring torque through shearing force of an oil film between power rollers and input and output discs, the toroidal CVT uses traction oil, which is large in friction loss during engagement of gears.
Moreover, sharing of oil increases a moving amount of oil in the power transmission system, leading to difficult establishment of the oil level.
It is, therefore, an object of the present invention to provide power transmission systems comprising a toroidal CVT and a power distributing device, which allow a reduction in friction loss of the power distributing device and in moving amount of oil in the power transmission system.
The present invention provides generally a power transmission system for a motor vehicle with an engine and front and rear wheels, which comprises: an input shaft which inputs power from the engine; a toroidal CVT arranged on the input shaft, the CVT being able to continuously convert power from the input shaft; a countershaft arranged parallel with the input shaft, the countershaft transferring power output from the CVT to the wheels; a power distributing device which distributes power from the countershaft to the front and rear wheels; a first casing which defines a first compartment, the first compartment accommodating the CVT; a second casing disposed adjacent to the first casing and defining a second compartment, the second compartment accommodating the power distributing device; a separation wall arranged between the first compartment and the second compartment, the separation wall sealing the compartments in a fluid-tight manner; and first and second oils charged in the first and second compartments, the first and second oils having different characteristics.